Dial Indicator 2020 Mount Backplate

Added some tabs to clip around 20mm v-slot extrusion.

Three versions are available: one with the tabs in the center, another with the tabs on the top to let it reach further down under the extrusion, and a third with a kind of "hook" to go over the top of the extrusion, in case you use a 2040 extrusion. **I have not tested the hook version yet to see how well it grips the extrusion.**

As I tightened the screws on the back of the dial with the center tabs version, I noticed the two tabs pinching together like tweezers. I figure this will help it grip the extrusion better.